Caption: An orange calcite crystal and crystals of galena found at Joplin, Missouri in the USA. Galena (lead sulphide) usally occurs as compacted masses of cubic lead-grey crystals, such as these, although some possess a rarer octahedral form. A common mineral appearing in various types of rocks, it is the most important ore of lead and is found with silver as a frequent by-product. Calcite (calcium carbonate) is a typical sedimentary mineral which usually occurs in large masses.